An Internet Protocol (IP) Multimedia Subsystem (IMS) is an IP connectivity and service control architecture that enables various types of media services to be provided to IMS endpoints using common Internet-based protocols. The IMS is access-independent in the sense that it can use access networks of various types to transport media signaling and bearer traffic. Once an IMS endpoint registers with an IMS network so as to inform the IMS network that it is ready to make and receive session requests, it can both initiate and terminate sessions for any type of media service.
The IMS architecture is defined in terms of functional entities (or simply “functions”). Functions constitute applications that are executed by a physical host or server. Different types of functions are logically grouped into separate layers or planes. The IMS media plane includes functions that actually transport and deliver media services so as to handle content traffic. The IMS control plane, by contrast, includes functions that control the media services so as to handle signaling traffic. The control plane for example includes functions that register an IMS endpoint with the IMS network and that control the establishment, maintenance, or tearing down of sessions. Exemplary IMS control functions in this regard include a Proxy Call Session Control Function (P-CSCF), an Interrogating Call Session Control Function (I-CSCF), and a Serving Call Session Control Function (S-CSCF). Other IMS control functions include a media resource function controller (MRFC) and a media gateway controller function (MGCF). See 3GPP TS 23.228 v13.0.0, incorporated by reference herein, for details about these well-known IMS control functions.
An operator of an IMS network may deploy multiple instances of the same IMS control function in the network to realize advantages such as increased signaling capacity, quality of service, and/or redundancy. Deploying multiple instances of the S-CSCF and/or P-CSCF for instance allows the IMS network operator to better handle peaks in the number of media sessions established. When the multiple S-CSCF and/or P-CSCF instances are deployed on hosts geographically distributed throughout the IMS network, the instance deployed on a host that is physically closest to an IMS endpoint will reduce the latency of the signaling path for that IMS endpoint.
Deploying multiple instances of the same IMS control function introduces complexities to the IMS network because the network must dynamically decide which instance to assign to a particular IMS endpoint. Known approaches focus on assigning the instance that will balance the load across the instances and/or their physical hosts, that provides resiliency to instance and/or host failure, and/or that is geographically closest to the IMS endpoint so as to minimize the end-to-end signaling delay. Tirana, P. & Medhi, D. (2010), Distributed approaches to S-CSCF selection in an IMS network., in ‘NOMS’, IEEE, pp. 224-231. These known approaches, however, fail to account for factors that dictate whether the instance assignment is actually optimal from an environmental perspective.